The discovery that certain ceramic materials, particularly copper oxides, exhibit superconductivity at above liquid nitrogen temperatures has stimulated intensive research. Many uses for such materials have been suggested and attempted, including, for example, devices operating with microwave or radio frequency signals such as antennas, magnetic resonance imaging pickup coils, resonators, and the like. Optimal performance of such devices may depend upon having the lowest possible surface resistance.
Low-surface resistance high-temperature superconducting materials have been successfully fabricated in the form of thin films of ceramic. Such films typically have a thickness on the order of 0.5 micron and are formed by depositing the ceramic material or its precursors on the surface of a planar, single crystal substrate using techniques such as co-evaporation, sputtering, laser ablation, and molecular beam epitaxy. The resulting thin films have a single crystal structure, which, owing to the absence of grain boundaries, exhibits a low-surface resistance. The techniques which are used to deposit a thin film of superconducting material are, however, expensive and require careful control. Moreover, it has proved extremely difficult, if not impossible, to form single crystal thin films of any appreciable size or to prepare such films on curved substrates such as fibers or tubes.
The techniques available for the production of superconducting "thick" films, i.e. films having a thickness greater than one micron and typically in the range of from 10-200 microns, from ceramic materials are relatively easy to control and enable films of relatively large surface area to be prepared on both planar and curved substrates. Such superconducting thick films may be formed by sintering or "melt-texturing" of a fully reacted or calcined ceramic material which has been deposited on a substrate. Under those processes, particulate starting materials may be blended together under high-shear conditions in order to produce a finely divided, homogeneous powder blend. This blending of the starting materials may be effected on a dispersion thereof in a liquid medium such as ethanol, which is then removed after the blending operation is complete. The constituents of the resulting powder blend are than reacted together by firing (a process called calcining), the temperature and duration of the firing process being selected so as to fully convert the starting materials into the ceramic material. After milling, the calcined ceramic material is formulated into a coating composition or ink, together with a vehicle which typically comprises a solution or dispersion of an organic polymeric material in a liquid solvent or dispersant. The coating composition is then used to deposit a layer of the ceramic material onto the substrate using techniques such as screen printing, painting, doctor blading and spin coating. Drying and sintering of the so-deposited layer at above the peritectic temperature (the temperature at which, upon cooling, a liquid phase combines with a solid phase to form a new solid phase) for the ceramic material yields the superconducting thick film.
As the material cools after sintering below the peritectic temperature, the microstructure consists of a random array of crystallites of small grain size. Films sintered at temperatures at or above the peritectic temperature consist of larger grains and possess a certain amount of preferred orientation, i.e. with the c-axis perpendicular to the substrate surface. Generally, the larger the grain size, the lower the surface resistance in the resultant superconducting thick film. The formation of larger grains may be accomplished by sintering the superconductor material above its peritectic temperature and cooling it relatively slowly. Increasing the melt time or slowing down the cooling process may be problematic, however, because the substrate may tend to react undesirably with the superconducting material. As a result, melt or sinter times, cooling times, and temperatures are limited, leading to superconducting thick films with many grain boundaries and having higher surface resistances.